The present invention relates generally to optical communications networks. More particularly, the present invention relates to an optical amplifier with a very broad bandwidth and to optical communications networks incorporating the same.
Wavelength-division-multiplexed (WDM) optical communications networks support the transmission of a number of different peak wavelength optical signals in parallel on a single optical fiber. Each one of such signals, typically referred to as a channel, represents an independent data stream.
FIG. 1 depicts a simplified schematic diagram of a typical WDM network 100 in the prior art. It will be clear to those skilled in the art that a typical WDM optical communications network will have many other elements than are depicted in FIG. 1. These other elements are not shown so that attention can be focused on those elements that are germane to an understanding of the present invention.
WDM network 100 includes a plurality of transmitters TX-1 through TX-n, each of which transmitters includes an optical source for generating an optical signal xcex-i, i=1, n. Each optical signal xcex-i has a unique peak wavelength onto which information is modulated in well-known fashion. The plurality of optical signals xcex-1 through xcex-n are combined into a single xe2x80x9cmultiplexedxe2x80x9d signal m-xcex by wavelength multiplexer 102 in known fashion. Multiplexed signal m-xcex is then launched into optical fiber 104.
Nodes 106 and 108 are in optical communication with WDM network 100 and are operable to receive multiplexed signal m-xcex. Such nodes comprise subscriber terminals (e.g., subscriber terminals 106-S1 and 108-S1, etc.) each having a receiver(s) (not shown) for receiving at least some of the information carried by multiplexed signal m-xcex.
To provide the requisite information to an individual subscriber terminal, nodes 106 and 108 typically include a means for removing or separating the channel(s) carrying such information from the other channels of multiplexed signal m-xcex. Depending on the size of the node (ie., the number of subscriber terminals, etc.), an add-drop filter (not shown) or a demultiplexer, such as demultiplexer 110, may suitably be used to remove one or more appropriate channels from multiplexed signal m-xcex. The removed channel(s) are then delivered to the appropriate subscriber terminal(s).
Multiplexed signal m-xcex is attenuated as it transmitted over the WDM network 100. Such attenuation is due, for example, to losses that occur as the signal propagates through waveguides (e.g., optical fiber 104) and as it passes through add-drop filters (not shown) or demultiplexers at network nodes. To compensate for signal attenuation, WDM network 100 includes in-line optical amplifiers 112 for supplying additional gain to multiplexed signal m-xcex.
Optical amplifiers 112 for WDM networks are typically implemented as fiber amplifiers. In fiber amplifiers, an optical pump excites doping ions to a higher energy level from which amplification takes place by stimulated emission. The doping ions are rare-earth elements such as erbium, praseodymium, and neodynium. As an alternate to doping, amplification can also be provided using Raman scattering. In Raman scattering, a small portion of an incident frequency is converted into other frequencies. The effect can be used to transfer energy from a pump laser to a weak signal.
Rare-earth doped fiber amplifiers have an optical bandwidth of about 80 nanometers (nm) (at 15-30 dB gain) and Raman fiber amplifiers have an optical bandwidth of about 50 nm (at 10-15 dB gain). Such bandwidths may be acceptable in some systems, or may suitably be broadened piecewise as desired by combining several such amplifiers. But ultra-broadband continuous gain, such as will be necessary in future optical networks, cannot presently be obtained with fiber amplification.
Another way to implement an optical amplifier is as a semiconductor amplifier wherein gain is provided by stimulated emission from injected carriers. Semiconductor amplifiers are not, however, typically used in WDM systems because they induce cross-signal modulation by spectral hole burning and four-wave mixing. These effects can be avoided by separating the spectral components (i.e., the various wavelength signals) that comprise the multiplexed signal and directing them to different semiconductor amplifiers. Such separation has required the use of fiber devices (e.g., waveguide routers, etc.). The use of such devices increases signal attenuation and increases system cost.
The art would therefore benefit from an amplifier that provides ultra-broadband gain but avoids the drawbacks of the prior art.
An article comprising a broad band amplifier that avoids the drawbacks of the prior art is disclosed. In one embodiment, the broad band amplifier comprises a free-space wavelength demultiplexer/multiplexer and optical gain means.
The free-space wavelength demultiplexer/multiplexer is operable to receive a multiplexed signal and to demultiplex it into its constituent spectral components. Each such spectral component is characterized by a different peak wavelength. The free space wavelength demultiplexer/multiplexer(s) is advantageously implemented as a grating demultiplexer/multiplexer. In one embodiment, the grating demultiplexer/multiplexer is realized via an arrangement that includes collimating/focusing optics and a plane grating.
The optical gain means comprises a plurality of xe2x80x9cwavelength-specificxe2x80x9d gain regions. In the present context, the phrase xe2x80x9cwavelength-specificxe2x80x9d means that an individual gain region is operable to impart gain over a specific, narrow range of wavelengths. A given gain region is advantageously operable over a different wavelength range than all other gain regions. In some embodiments, the gain regions comprise semiconductor optical amplifiers.
In some embodiments of the present invention, the free-space wavelength demultiplexer/multiplexer is operable to deliver each spectral component, as a function of its wavelength, to the xe2x80x9ccorrespondingxe2x80x9d wavelength-specific gain region. In the present context, the term xe2x80x9ccorrespondingxe2x80x9d means that the delivered spectral component is characterized by a peak wavelength that is banded by the operating range of the gain region that receives that spectral component. As a consequence, the spectral components are amplified. Since spectral components are physically separate and are amplified in physically separate gain regions, the cross-signal modulation that is prevalent in prior art semiconductor optical amplifiers is avoided.
After amplification, the spectral components are re-multiplexed. In some embodiments, this is accomplished by reflecting the spectral components toward the free-space wavelength demultiplexer/multiplexer. In such a case, the path taken by the amplified spectral components through the free-space wavelength demultiplexer/multiplexer is the reverse of the path followed by the spectral components before amplification. As such, during the reverse pass, the free-space wavelength demultiplexer/multiplexer (re)multiplexes the spectral components.
In other embodiments, rather than reflecting the amplified spectral components to the free-space wavelength demultiplexer/multiplexer, they are delivered to a second free-space wavelength demultiplexer/multiplexer. The second free-space wavelength demultiplexer/multiplexer is operable to (re)multiplex the spectral components and launch the resulting multiplexed signal back into a waveguide for transmission through a WDM network. In both cases, with the exception of the imparted gain, the reconstituted version of the multiplexed signal is identical to the original un-amplified multiplexed signal.